To afford digital communication among electronic components in automotive vehicles a class 2 bus has been proposed. Such a bus would serve many disparate modules by carrying pulse width modulated signals over a common line comprising a single wire. While digital signals are commonly expressed by square wave pulses, the fast rise and fall times give rise to electromagnetic interference (EMI) which is especially undesirable in the class 2 system due to the opportunity to emit radiation from the extended line. To avoid such radiation each transmitter module filters or shapes the square wave data input pulses to assure that for the corresponding transmitted pulses the rise and fall times are slew rate limited and wave corners are rounded rather than sharp, thereby avoiding rapid changes of signal current.
The many vehicle components served by the bus have their own power supplies, and while grounding through the vehicle structure may maintain some degree of commonality, there remains a variance of ground voltages among the various modules. Since each module is coupled to the common line via resistive and capacitive impedance, they all affect the ground potential of the line itself. As a result, the line voltage may appear to a given module to be as much as 2 volts above or below the ground voltage of the module. If then, a carefully shaped voltage pulse is applied to the line at a much different ground voltage, the pulse current will be distorted to include rapid current changes, especially at the inception of the pulse, to cause EMI.